There are many facilities, buildings, sites and locations that require electric power and yet do not have fixed electric power distribution systems. For example, in the entertainment industry, motion picture and television production studios typically comprise a building shell to provide an interior that is isolated from ambient light and sound. The building shell does not contain any fixed internal structures or utilities to afford maximum versatility in the use of the interior of the building shell for the construction of sets. As should be understood, sets for motion picture productions vary widely from film project to film project and the electrical requirements for any particular set are not known until the project is scheduled for production and the set is designed. Any fixed internal electrical wiring or power distribution equipment would limit the placement of lighting fixtures, technical equipment and other devices, such as special effects machines, and, thus, restrict the use of the studio. Accordingly, portable power distribution arrangements are installed to service the power needs of a particular set and are then removed when the set is struck.
There are also many other sites and locations having power distribution needs that vary with time, such as an outdoor site used for traveling carnivals, outdoor concerts, theme parks and so on. Indeed, there are a great many locations that require a portable power distribution facility that can be easily installed on a temporary basis, configured and reconfigured to any desired network of main power feed, trunk feeds and branch circuits and conveniently disassembled for storage and/or shipment to other locations. Examples of such locations, in addition to those discussed above, include construction sites, convention and exhibit halls and any area or location during times of emergency such as earthquakes and the like.
However, presently known components used to provide portable power distribution are heavy and massive in size and construction, are not standardized, and are generally difficult and labor-intensive to install. They are also not suitable for convenient handling, storage or transportation to various sites. For example, the motion picture industry typically utilizes heavy panel boards, each containing a main connection to a power source. Each panel board includes power outputs having large machine-screw studs to which lugged cables are attached. Over current protection is generally not provided for any of the branch circuits fed by the lugged cables. Moreover, the panel boards are so large and heavy that they require machines to lift and load them into a truck for transportation.
The motion picture industry also commonly utilizes a wiring device referred to as a "Sister" lug. A sister lug generally comprises a cast brass lug with a U shaped opening and a set screw. The sister lug is typically soldered to a flexible cable for transmission of electrical power to or from the lug. In practice, the U shaped opening of the sister lug is received around a cooper conductor bus of a power cabinet input or output terminal and held in contact with the bus by the set screw. In this manner, electrical power can be transmitted to and from a power cabinet via an arrangement of cables and sister lugs.
While sister lugs provide a fast, rugged and inexpensive form of electrical connector, they pose electrical code compliance problems. For example, the set screw of the sister lug arrangement is used to secure the lug to the bus and to provide a good electrical contact between the lug and the conductor bus. Accordingly, as required by, for example, the National Electric Code (NEC), an ancillary strain relief device must be provided to relieve any mechanical force applied to the cable soldered to the lug.
Without an ancillary strain relief, mechanical forces would be applied directly to the set screw which could cause the electrical connection to loosen. Indeed, if the mechanical force is great enough, the sister lug may become free from the connector bus. In addition, an electrical overload on the cables or electrical arcing caused by a loose connection can heat the solder connection between the cable and the lug. In some circumstances, the heat generated can be sufficient enough to melt the solder connection and free the cable. The use of an ancillary strain relief device makes certain that the cable is secured in a fixed position in the event the sister lug loosens or the cable becomes disconnected from the lug due to, e.g., solder melting.
However, the use of an ancillary strain relief device adds to the amount of labor needed to attach a sister lug to a conductor bus. When many lugged cables must be connected in a power distribution network, the total extra labor required for securing ancillary stress relief devices individually to each cable in the network can become a considerable disadvantage in the use of sister lugs. Furthermore, the use of conventional electrical cable clamps for ancillary strain relief is ordinarily not feasible inasmuch as the typical size of a sister lug exceeds the diameter of a conventional cable clamp.
In another known portable power device, generally referred to as a load splitter box, several bus bars are mounted on insulators. Each bus bar is coupled to a lug-connector input of a heavy feeder cable. C-clamp type lugs are used to connect branch circuit cables to the bus bars and no over-current branch protection is provided for the individual branch circuit cables. Carnivals often use such load splitter boxes and simply cover exposed electrical connections with a rubber mat. Such load splitter boxes are labor-intensive to install and do not accommodate a wide variety of electric power applications.
In addition, the lack of over-current protection for each individual branch circuit coupled to the panel board or load splitter box, is highly undesirable. For example, a branch overload or short circuit can result in dangerous cable overloading and resultant fires. There are also inherent hazardous conditions present in a wire or cable size reduction, as for example, from the bus bar to branch circuit cable of the load splitter box. Such potential hazardous conditions remain unchecked without overcurrent protection for each individual branch circuit.
Moreover, in many instances, various power distribution units must be coupled to one another to provide a power distribution network. The lack of standardization between known power distribution devices and the varying approaches to portable power distribution adapted on an "as-needed" basis by those who require portable power distribution, has left the industry without a cost-effective, convenient and versatile component suitable to provide a wide variety of network configurations supplying widely varying electrical needs.